1. Field of the Invention
The present invention relates to a converter for converting a state quantity of, for instance, a pressure of a liquid or a gas, an acceleration, or a temperature to other type of state quantity such as an electrical signal.
2. Description of the Related Art
As a converter for converting one type of state quantity to other type of state quantity, there has been known, for instance, a pressure sensor for converting a pressure to an electrical signal.
FIG. 10 and FIG. 11 are a general perspective view and an exploded perspective view each showing a pressure sensor 110 based on the conventional technology.
The pressure sensor 110 comprises an upper substrate 130 and a lower substrate 140, each made from an insulating material such as glass; a variable electrode 120 which can be displaced against the upper and lower substrates 130, 140 and is made from silicon; and a fixed electrode 131 provided on a detection face 130A of the upper substrate 130 opposing to this movable electrode 120.
As shown by the two-dot chain line in FIG. 11, the fixed electrode 131 comprises a central electrode 132 provided at a substantially central portion of the detection face 130A of the upper substrate 130, and a peripheral electrode 133 surrounding the central electrode 132.
The movable electrode 120 is electrically connected to a first signal fetching section 122 extending from a side face 121C to an upper face 130B of the upper substrate 130.
As the movable electrode 120 is made from silicon, the first signal fetching section 122 is made from aluminum which can form an electrically ohmic conjunction with and also be wire-bonded to silicon.
The central electrode 132 and the peripheral electrode 133 are made from titanium having the corrosion resistance, and are electrically connected to second signal fetching sections 134, 135 made from aluminum on the upper face 130B (a surface opposite to the detection surface 130A) of the upper substrate 130 via through-holes 136, 137.
With the configuration as described above, when a pressure to be measured is loaded to the surface of the movable electrode 120 opposite to the lower substrate 140, the movable electrode 120 bends, and an electrostatic capacitance between the movable electrode 120 and the central electrode 132 or between the movable electrode 120 and the peripheral electrode 133 changes. By electrically connecting the electrodes 120, 132, 133 to a signal processing circuit via the signal fetching sections 122, 134, 135, it becomes possible to accurately measure the pressure to be measured by measuring changes in the electrostatic capacitances between the central electrode 132 and the movable electrode 120 and between the peripheral electrode 133 and the movable electrode 120.
When the converter 110 is used under the existence of corrosive gas, the corrosive gas is present also around each of the signal fetching sections 122, 134, 135, and the aluminum forming the signal fetching sections 122, 134, 135 may disadvantageously be afflicted by the corrosive gas.
To solve the problem, there may be the possibility of forming the signal fetching sections 122, 134, 135 with a metal having the high corrosive resistance against the corrosive gas such as, for instance, titanium. Although an excellent ohmic conjunction with silicon having a low content of impurities can not be obtained by using titanium, it is possible to obtain a good ohmic conjunction with silicon containing a large quantity of impurities. On the other hand, when the content of impurities of the silicon becomes higher, the specific resistance becomes lower, and also the adaptability to wet-etching with an alkali solution becomes disadvantageously lower.
For instance, there has been known the converter in which only a portion of the movable electrode contacting the first signal fetching section contains impurities (Refer to Japanese Patent Laid-Open Publication No. HEI 08-94472).
With the configuration, it is possible to obtain a good ohmic conjunction with the first signal fetching section in a portion of the movable electrode contacting the first signal fetching section, and also it is possible to prevent the adaptability to etching from being degraded in portions thereof other than that contacting the first signal fetching section.
However, in the converter in which a portion of the movable electrode contains impurities, as the impurities are locally introduced by means of the impurities dispersing method or the like in the process of manufacturing a silicon wafer forming the movable electrode, the number of production steps increases, which results in increase of production cost of the converter.